Internal combustion engines on the basis of Otto (gasoline) engines are generally operated with fuel from hydrocarbons, from fossil fuels based on refined crude oil. Alcohol produced from renewable resources (plants), for example, ethanol or methanol, is increasingly being added in various mixing ratios to this fuel. In the USA and Europe, a blend of 75-85% ethanol and 15-25% gasoline is often utilized under the trade name E85. The internal combustion engines are designed in such a way that they can be operated with pure gasoline as well as with blends up to E85. This is denoted as a “flex-fuel operation”. The operating parameters in the flex-fuel operation have to be adapted in each case to the existing fuel blend for an efficient operation with only a small discharge of toxic emissions; while at the same time high engine performance is guaranteed. A stoichiometric air-fuel ratio is, for example, present at 14.7 mass parts of air per part of gasoline; however, when using ethanol, a proportion of air of 9 mass parts must be set.
The current fuel composition before the point of injection time and the current exhaust gas composition, therefore the partial pressure of the oxygen in the exhaust gas, are determined via the interaction of sensors, and are conveyed to the control electronics of the internal combustion engine. Based on this sensor-data, the combustion of the internal combustion engine is optimized, especially via the setting of the ignition timing and the most advantageous air-fuel ratio.
Various fuel composition sensors are utilized to determine the composition of the fuel blend. Fuel composition sensors use the various properties of alcohol and gasoline for determining the composition of the fuel. Therefore, ethanol, for example, is a protic solvent, which contains hydrogen ions and has a large dielectric constant, which is, however, dependent on the water content. Gasoline, on the other hand, is an aprotic solvent with a small dielectric constant. Based on this, there are fuel composition sensors, which determine the fuel composition with the aid of the dielectric properties of the fuel blend. Other fuel composition sensors use the differing electric conductivity or the differing optical properties of the fuels, like, for example, the differing refractive indexes.
Furthermore, systems for determining the fuel composition are known, which do not use specific fuel composition sensors, but which evaluate the signals of the existing sensors in the internal combustion engine. With these systems as well as with systems using fuel composition sensors, a mixture adaptation is carried out, which corrects systematic mixture errors, that can occur because of air leakage or railpressure-offset. Multiplicative mixture errors, which have an equal effect in the entire load speed range, can, in the steady state operation of an engine at operating temperature, initially not be distinguished from mixture deviations due to a modified mixing ratio. A modified stoichiometric factor of the air/fuel mixture is consequently not detectable in this case, and an incorrectly working ethanol sensor would not be detectable and would lead to a trimming of the mixture adaptation.
In systems without an ethanol sensor, the determination of the fuel mixing ratio takes place by means of a fuel adaptation. When a fuel adaptation takes place, the mixture adaptation is temporarily discontinued after fueling, and the fuel-tank ventilation is blocked, so that an undefined amount of fuel via the intake air can not falsify the fuel adaptation. Subsequently, the fuel mixing ratio of ethanol and gasoline can be determined, for example, via the proportion of air and fuel required for a stoichiometric combustion.
Especially a temporary, but systematic mixture deviation present during the fuel adaptation leads to a divergence of the adaptation values. An incorrectly adapted fuel mixing ratio can likewise mistakenly be adapted as a mixture error in the mixture adaptation; or a mixture error in the fuel adaptation, which occurs during the fuel adaptation, can be mistakenly adapted as a fuel mixing ratio. A fuel mixing ratio, which is falsely determined, does not, in fact, have any influence on the air/fuel ratio lambda in the steady state operation of an engine at operating temperature. It does, however, erroneously affect the settings of the engine management system for the calculation of the angular ignition spacing, for the cold start behavior and for optimizing the degree of efficiency.
The task of the invention is to provide a method, which makes a reliable and cost-effective detection of the composition of a fuel blend of at least two fuels possible.